I think as a mini-game it would be fun to introduce some sort of small-duration RTS within WoW, using your characters interacting with prominent lore characters etc. Like, say, you have a mission to strike an Orc outpost in Ashenvale, Shandris Feathermoon is under your control, along with your toon, and additional Sentinels. It'd be a stretch though, people would end up complaining about it too. Puzzles and other thinking games as mini-games could also be very interesting. Especially with all the Ancient races in game, having some class or even racial based questline following a combination of scenarios, instances, puzzles, and normal questing could be very engaging.

While I have my doubts regarding this idea, player-created content is an extremely unexplored area in WoW and should be focused on more. It only really exists as a major factor in the form of world PvP and on some role-playing realms. Luckily it seems that the game is trying to take steps, albeit very small ones, towards the sandbox model.

It was done for City of Heroes/Villains, there was some good stuff, but it was very few and far between, with so much crap and broken stuff - massively.joystiq.com/2009/04/18/city-of-heroes-player-created-content-surpasses-devs-in-24-hour/

I honestly think if something like this happened in WoW, I believe the content created would need to tested in house and checked before being released to play publicly.